This invention relates to quadraphonic audio systems, and more particularly to systems for encoding four or more individual channels of audio information for broadcast over FM multiplex radio, and for decoding and reproducing the broadcast information either as four discrete audio output signals having the directionality of the original input signals or "semi-discretely" in the manner of matrix systems, the broadcast information being fully compatible with existing monophonic and stereophonic receivers. The subject matter of this application is related to subject matter in the co-pending application Ser. No. 462,042 entitled "Compatible Four Channel Recording And Reproducing System" filed of even date herewith and assigned to the same assignee as the present application.
The nature of the encoding employed in the "SQ" quadraphonic record system described in applicant's co-pending application Ser. No. 384,334 filed July 31, 1973, now U.S. Pat. No. 3,890,466, has made it possible to broadcast quadraphonic information over FM multiplex radio in a manner which is fully compatible with monophonic and stereophonic receivers. In a broadcast mode, the two composite signals transduced from an SQ record, each of which contains three of the four input signals with preselected amplitude and phase relationships, can be transmitted in the same manner as a conventional stereophonic signal pair. Upon reception, a conventional monophonic or stereophonic FM receiver gives the listener a totally satisfactory monophonic or stereophonic presentation, as the case may be. If an SQ decoder of the type described in applicant's co-pending application Ser. No. 338,691, filed Mar. 7, 1973, now U.S. Pat. No. 3,835,255, is used in combination with the stereophonic FM receiver, the listener can obtain four-channel reproduction of the four signals contained in the transmitted composite signals. It is a primary object of the invention to provide an improved system for broadcasting and receiving four independent channels discretely or semi-discretely, in a manner that is fully compatible with existing SQ decoders, as well as with existing stereophonic and monophonic receivers.
As further background for understanding the present invention and the differences between it and the more simplified method of SQ broadcasting alluded to above, it will be useful to discuss in further detail the characteristics and limitations of simply transmitting the two composite encoded signals. Employing the SQ system, the broadcaster can use an SQ encoder, of which several types are disclosed in applicant's co-pending application Ser. No. 384,334 filed July 31, 1973, for combining the four channels of a four-channel program, either live or taped, into two composite signals, and transmitting them through existing FM stereo transmitter equipment. This approach provides satisfactory results at the receiver as long as there are no soloists or important instruments placed in the center-back of the quadraphonic field. It is a characteristic of the SQ code that center-back signals are encoded in phase opposition and thus are not received through a monophonic receiver; however, they are reproduced normally with stereophonic FM receivers and properly decoded when an SQ code does not significantly handicap a record producer because he can be careful to avoid placing soloists in the "dead back" of the audience, and indeed, it may be to his advantage when recording symphonic music in highly reverberant halls. In the latter case, the orchestra is picked up normally with a conventional microphone array for transmission over the two front channels, while the reverberant energy received with additional microphones is distributed among the remaining three channel pairs so as to convey to the quadraphonic listener a realistic impression of the sounds of the hall. For the monophonic listener, however, with all the music and a full measure of reverberation concentrated in one loudspeaker, the reverberation tends to mask the fine structure of the music, producing "muddy" sound. With the basic SQ code the producer is able to concentrate a greater measure of reverberant energy predominantly between the back channels, thereby to cause it to be diminished in the monophonic mode to give a more satisfactory energy balance. The stereophonic listener, of course, hears all the signals but the anti-phase reverberant energy becomes distributed, at least in part, beyond the confines of the loudspeakers thus preserving a satisfactory direct/reverberant signal balance for the main-front stage.
Unfortunately, the broadcasting program director often does not have the opportunity to carefully plan and edit the material being encoded for broadcast. He may have to transmit any four-channel program through the SQ system, for example, from an existing tape, which might contain a center-back soloist; or a live rock concert during the broadcast of which an artist might wander between the back-channel microphones. To accommodate these contingencies, the broadcast producer can use the "forward-looking" encoder illustrated in FIG. 20 of the aforementioned co-pending application Ser. No. 384,334 which treats the center-back signal in an in-phase manner as if it were a center-front signal. Using this encoder, the sounds around the front and sides of the quadraphonic field, including left-back, left-front, center-front, right-front, and right-back channels are ideally coded, the signals between the back channels are redistributed among all the channels, and the center-back signal is moved forward to the center-front position. With this code, then, all listeners, monophonic, stereophonic and quadraphonic, hear all of the signals, but the quadraphonic listener perceives center-back signals as if they originated at center-front. In any event, it will be appreciated that the quadraphonic listener, using a matrix decoder, does not hear four discrete signals; each is "contaminated" to some degree with lower level signals from two other channels, as is inherent in "matrix" four-channel systems.
Although the above-described SQ system of broadcasting provides generally acceptable quadraphonic reproduction at the receiver, which can be further improved to approach the quality of discrete reproduction through the use of logic and control circuitry in conjunction with the matrix decoder, there is considerable current interest in providing a system for broadcasting four channels in a manner which allows fully discrete presentation at the receiver. Again, such a system should still retain full compatibility with conventional stereophonic and monophonic receivers and be capable of operation on frequency channels allocated by the Federal Communications Commission. To achieve these ends, one can first appropriately combine the four signals of the four-channel program into a two-channel mono-compatible stereophonic program for transmission over existing FM broadcast channels and also provide for transmission of at least two additional sets of signals which, upon reception followed by appropriate combination of received signals, will enable restoration of the original four program signals. In one proposed system of which applicant is aware, four audio signals are combined to form a pair of signals designated (L.sub.f + L.sub.b) and (R.sub.f + R.sub.b) and the sum and difference of these signals are transmitted by an FM multiplex transmitter. The four program signals are also combined into two additional signals (L.sub.f + R.sub.f) - (L.sub.b + R.sub.b) and (L.sub.f + R.sub.b) - (R.sub.f + L.sub.b) which are transmitted simultaneously as modulation on additional sub-carriers. Upon reception, the four sets of signals are appropriately recombined to derive the original four audio signals L.sub.f, R.sub.f, L.sub.b and R.sub.b ; such recombination being possible as long as the sets are non-trivial and are linearly independent. A number of systems employing this basic scheme of signal combination, including the system described in Dorren U.S. Pat. No. 3,708,623, are currently being evaluated by the NQRC (National Quadraphonic Radio Committee). It is unnecessary for present purposes to discuss the merits of the above-described sets of signals, except to point out that each of them statistically carries equal amounts of power with the consequence that it is necessary to transmit a relatively high total signal energy to perform the "discretizing" operation.
While the advantages to be derived from "discrete" quadraphonic broadcasting are at this time inconclusive, and a variety of proposals are being considered, it would be disirable, in the event that a discrete system of quadraphonic broadcasting is adopted, that such systems be compatible with SQ decoders and decoder-equipped radio receivers currently in the hands of consumers. It is an object of the present invention to provide a four channel system of broadcasting which allows fully discrete reception, but which is also compatible with existing monophonic, stereophonic and SQ receiving and reproducing equipment. SUMMARY OF THE INVENTION
The present invention is directed to a compatible four channel system for use in conjunction with a radio transmission system for transmitting four individual audio signals designated L.sub.f, L.sub.b, R.sub.b, and R.sub.f over a medium having primarily and secondary information channels and first and second subsidiary information channels, the primary and secondary channels carrying information that is consistent and compatible with existing monophonic and stereophonic standards. In accordance with the preferred embodiment of the invention, means are provided for forming a first composite signal designated L.sub.T which contains, to the extent they are present, L.sub.f in a dominant proportion and L.sub.b and R.sub.b in sub-dominant proportions, L.sub.b and R.sub.b being phase shifted with respect to each other. Means are also provided for forming a second composite signal designated R.sub.T which contains, to the extent they are present, R.sub.f in a dominant proportion and L.sub.b and R.sub.b in sub-dominant proportions, L.sub.b and R.sub.b being phase shifted with respect to each other. First and second conjugates of the first and second composite signals, respectively, are formed, the first and second conjugates being designated L.sub.T.sup.* and R.sub.T.sup.*. Means are provided for forming a sum signal as a function of the sum of L.sub.T and R.sub.T and for applying the sum signal to the primary information channel. Means are also provided for forming a difference signal as a function of the difference between L.sub.T and R.sub.T and for applying the difference signal to the secondary information channel. Finally, means are provided for applying the first and second conjugates, L.sub.T * and R.sub.T *, to the first and second subsidiary channels, respectively. In accordance with the preferred embodiment of the invention, a receiver/decoder is provided which includes means responsive to the primary and secondary channels for processing the sum and difference signals to obtain the first and second composite signals, L.sub.T and R.sub.T. L.sub.T and L.sub.t * are combined to recover the individual audio signal L.sub.f. R.sub.T and R.sub.T * are combined to recover the individual audio signal R.sub.f. Also, means are provided for combining relatively phase shifted versions of the composite signals and the conjugates to recover the third and fourth individual audio signals, R.sub.b and L.sub.b.
The original program signals can be encoded in a variety of ways to provide signals at the receiver necessary to accomplish the discretizing function, several of these ways being described hereinbelow. A feature of each of the described embodiments is that the signal energy in the auxiliary signals carrying the information necessary to accomplish descretizing is significantly lower than the signal energy in the channels carrying the principal information. As a consequence, the total signal energy that must be handled by the transmitter is lower than that required for other systems that have been proposed for discrete broadcasting of four channel program information.
Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.